Fix markup typo

Author: SanMuHe

linux/list-directories-in-tree-like-format.md

@@ -6,6 +6,6 @@ For example,
 - `tree` displays the contents of the current directory and subdirectories in a tree-like format.
 - `tree -P 't*'` displays only directories beginning with `t` or directories containing files that begin with `t`.
 - `tree -d` displays directories only.
-- 'tree -L 2' restricts the max depth of the subdirectories to be 2.
+- `tree -L 2` restricts the max depth of the subdirectories to be 2.
 
 Note: if your get "command not found" error when run the tree command, you can install it with `sudo apt-get install tree`.